We have extensively characterized prosolin, a major cytosolic protein of proliferating HL-60 promyelocytic leukemia cells that undergoes very rapid phosphorylation in response to treatment of the cells with tumor-promoting phorbol esters like TPA as an early step in a pathway leading to cessation of cell growth and onset of monocytoid differentiation. Prosolin has been identified in proliferating human peripheral blood lymphocytes (PBL) by tryptic peptide mapping of proteins isolated from two-dimensional electrophoretic gels. While virtually absent from resting PBL, prosolin expression is induced by mitogens as a late event, during the onset of S-phase. Its level of synthesis correlates with the prevalent level of DNA synthesis. Prosolin undergoes rapid phosphorylation in proliferating PBL in response to TPA or A23187 treatment, together with an inhibition of DNA synthesis. Phosphorylation of prosolin may be part of the mechanism by which naturally occurring growth inhibitory factors terminate normal lymphocyte proliferation. T-cell leukemia cell lines show a marked reduction in the phosphorylation of prosolin in response to A23187 and other agents. We are cloning and sequencing cDNAs to prosolin. Partial amino acid sequence data were obtained by analysis of tryptic peptides of prosolin isolated from two-dimensional electrophoretic gels. Degenerate oligonucleotide probes were synthesized and used to probe a lymphoblast cDNA library.